Additive for increasing the density of a fluid and fluid comprising such additive

ABSTRACT

An additive which increases the density of wellbore fluids used during the construction or repair of oil, gas, injection, water or geothermal wells comprises solid colloidal particles of weight average particle diameter (D 50 ) of less than 2 microns, the particles being deflocculated by the action of a dispersant, preferably incorporated during the process of grinding or communication of the particles to the specified particle size. The additives may be used in any wellbore fluid such as drilling, cementing, completion, packing, work-over (repairing), stimulation, well killing and spacer fluids as well as in a dense media separating fluid or in a ship&#39;s ballast fluid.

This invention relates to products which increase the density ofwellbore fluids used during the construction or repair of oil, gas,injection, water or geothermal wells. The products of this invention maybe used in any wellbore fluid such as drilling, cementing, completion,packing, work-over (repairing), stimulation, well killing, and spacerfluids.

One of the most important functions of a wellbore fluid is to contributeto the stability of the well bore, and control the flow of gas, oil orwater from the pores of the formation in order to prevent, for example,the flow or blow out of formation fluids or the collapse of pressuredearth formations. The column of fluid in the hole exerts a hydrostaticpressure proportional to the depth of the hole and the density of thefluid. High pressure formations may require a fluid with a specificgravity of up to 3.0.

A variety of materials are presently used to increase the density ofwellbore fluids. These include dissolved salts such as sodium chloride,calcium chloride and calcium bromide. Alternatively powdered mineralssuch as barytes, calcite and hematite are added to a fluid to form asuspension of increased density. It is also known to utilise finelydivided metal such as iron as a weight material. In this connection, PCTPatent Application WO85/05118 discloses a drilling fluid where theweight material includes iron/steel ball-shaped particles having adiameter less than 250 μm and preferentially between 15 and 75 μm. Ithas also been proposed to use calcium or iron carbonate (see for exampleU.S. Pat. No. 4,217,229).

It is a requirement of wellbore fluids that the particles form a stablesuspension, and do not readily settle out. A second requirement is thatthe suspension should exhibit a low viscosity in order to facilitatepumping and to minimise the generation of high pressures. Anotherrequirement is that the wellbore fluid slurry should exhibit lowfiltration rates (fluid loss).

Conventional weighting agents such as powdered barytes (“barite”)exhibit an average particle diameter (d₅₀) in the range of 10-30 μm. Tosuspend these materials adequately requires the addition of a gellantsuch as bentonite for water based fluids, or organically modifiedbentonite for oil based fluids. A soluble polymer viscosifier such asxanthan gum may be also added to slow the rate of the sedimentation ofthe weighting agent. However, a penalty is paid in that as more gellantis added to increase the suspension stability, the fluid viscosity(plastic viscosity) increases undesirably resulting in reducedpumpability. This is obviously also the case if a viscosifier is used.

The sedimentation (or “sag”) of particulate weighting agents becomesmore critical in wellbores drilled at high angles from the vertical, inthat sag of, for example, one inch (2.54 cm) can result in a continuouscolumn of reduced density fluid along the upper portion of the wellborewall. Such high angle wells are frequently drilled over large distancesin order to access, for example, remote portions of an oil reservoir. Inthis case it becomes even more critical to minimise a drilling fluid'splastic viscosity in order to reduce the pressure losses over theborehole length.

This is no less important in deep high pressure wells where high densitywellbore fluids are required. High viscosities can result in an increasein pressure at the bottom of the hole under pumping conditions. Thisincrease in “Equivalent Circulating Density” can result in openingfractures in the formation, and serious losses of the wellbore fluid.Again, however, the stability of the suspension is important in order tomaintain the hydrostatic head to avoid a blow out. The two objectives oflow viscosity plus minimal sag of weighting material can be difficult toreconcile.

The need therefore exists for materials to increase fluid density whichsimultaneously provide improved suspension stability and less viscosityincrease.

It is known that reduced particle sedimentation rates can be obtained byreducing the particle size used.

However, the conventional view in the drilling industry is that reducingthe particle size causes an undesirable increase in viscosity. This issupposed to be caused by an increase in the surface area of theparticles causing increased adsorption of water.

For example, “Drilling and Drilling Fluids” Chilingarian G. V. andVorabutor P. 1981, pages 441-444 states: “The difference in results(i.e. increase in plastic viscosity) when particle size is varied in amud slurry is primarily due to magnitude of the surface area, whichdetermines the degree of adsorption (tying up) of water. More water isadsorbed with increasing area.” Further it is also stated that“Viscosity considerations often will not permit the addition of any moreof the colloidal solids necessary to control filtration, unless thetotal solids surface area is first reduced by removing a portion of theexisting clays”. The main thrust of the argument is that colloidal finesdue to their nature of having a high surface area to volume ratio willadsorb significantly more water and so decrease the fluidity of the mud.This is why they and others have recommended that it is necessary inweighted particulate muds to remove the fine solids to reduce viscosity.The same argument or concept is presented in “Drilling Practices Manual”edited by Moore pages 185-189 (1986). Also, the API specification forbarite as a drilling fluid additive limits the % w/w below 6 μm to 30%maximum in order to minimise viscosity increases.

It is therefore very surprising that the products of this invention,which comprise particles very finely ground to an average particlediameter (d₅₀) of less than two μmmicrons, provide wellbore fluids ofreduced plastic viscosity whilst greatly reducing sedimentation or sag.

The additives of this invention comprise dispersed solid colloidalparticles with a weight average particle diameter (d₅₀) of less than 2μm and a defloculating agent or dispersant. The fine particle size willgenerate suspensions or slurries that will show a reduced tendency tosediment or sag, whilst the dispersant controlling the inter-particleinteractions will produce lower rheological profiles. It is thecombination of fine particle size and control of colloidal interactionsthat reconciles the two objectives of lower viscosity and minimal sag.

It is worth noting that small particles have already been used indrilling fluids but for a totally different purpose. Thus, EP-A-119 745describes an ultra high density fluid for blow-out prevention comprisedof water, a first and possibly second weighting agent and a gellant madeof fine particles (average diameter from 0.5 to 10 μm). The gellingagent particles are small enough to impart a good static gel strength tothe fluid by virtue of the interparticle attractive forces. On thecontrary, the present invention makes use of well dispersed particles:the interparticle forces tend to push away the other particles. If theconcentration of small dispersed particles is sufficient, no gellingagent is needed.

According to the invention, a dispersant is added to the particulateweighting additive to allow it to find an acceptable conformation on theparticle surface. This provides via a manipulation of the colloidalinteractions rheological control, tolerance to contaminants andmanipulation of the colloidal interactions rheological control,tolerance to contaminants and enhanced fluid loss (filtration)properties. In the absence of the dispersant a concentrated slurry ofthese small particles, would be an unpumpable paste or gel. According toa preferred embodiment of the present invention, the dispersant is addedduring the grinding or comminution process. This provides anadvantageous improvement in the state of dispersion of the particlescompared to post addition of the dispersant to fine particles. Thepresence of the dispersant in the comminution process yields discreteparticles which can form a more efficiently packed filter cake and soadvantageously reduce filtration rates.

According to a preferred embodiment, the dispersant is chosen so as itprovides the suitable colloidal inter-particle interaction mechanism tomake it tolerant to a range of common wellbore contaminants, includingsalt saturated.

According to a preferred embodiment of the present invention, theweighting agent of the present invention is formed of particles that arecomposed of a material of specific gravity of at least 2.68. This allowswellbore fluids to be formulated to meet most density requirements yethave a particulate volume fraction low enough for the fluid to bepumpable.

A preferred embodiment of this invention is for the weight averageparticle diameter (d₅₀) of the new weighting agent to be less than 1.5micron. This well enhance the suspension's characteristics in terms ofsedimentation or sag stability without the viscosity of the fluidincreasing so as to make it unpumpable.

A method of comminuting a solid material to obtain material containingat least 60% by weight of particles smaller than 2 μm is known forexample from British Patent Specification No 1,472,701 or No 1,599,632.The mineral in an aqueous suspension is mixed with a dispersing agentand then ground within an agitated fluidised bed of a particulategrinding medium for a time sufficient to provide the required particlesize distribution. An important preferred embodiment aspect of thepresent invention is the presence of the dispersing agent in the step of“wet” grinding the mineral. This prevents new crystal surfaces formedduring the comminution step from forming agglomerates which are not soreadily broken down if they are subsequently treated with a dispersingagent.

The colloidal particles according the invention may be provided as aconcentrated slurry either in an aqueous medium or an organic liquid. Inthe latter case, the organic liquid should have a kinematic viscosity ofless than 10 centistokes at 40° C. and, for safety reasons, a flashpoint of greater than 60° C. Suitable organic liquids are for examplediesel oil, mineral or white oils, n-alkanes or synthetic oils such asalpha-olefin oils, ester oils or poly(alpha-olefins).

Where the colloidal particles are provided in an aqueous medium, thedispersing agent may be, or example, a water soluble polymer ofmolecular weight of at least 2,000 Daltons. The polymer is a homopolymeror copolymer of any monomers selected from (but not limited to) theclass comprising: acrylic acid, itaconic acid, maleic acid or anhydride,hydroxypropyl acrylate vinylsulphonic acid, acrylamido 2-propanesulphonic acid, acrylamide, styrene sulphonic acid, acrylic phosphateesters, methyl vinyl ether and vinyl acetate. The acid monomers may alsobe neutralised to a salt such as the sodium salt.

It is known that high molecular weight polymers act as flocculants bybridging between particles while low molecular weight polymers forinstance (less than 10,000) act as deflocculants by creating overallnegative charges.

It has been found that when the dispersing agent is added whilegrinding, intermediate molecular weight polymers (in the range 10,000 to200,000 for example) may be used effectively. Intermediate molecularweights dispersing agents are advantageously less sensitive tocontaminants such as salt and therefore are well adapted to wellborefluids.

Where the colloidal particles are provided in an organic medium, thedispersing agent may be selected for example among carboxylic acids ofmolecular weight of at least 150 such as oleic acid and polybasic fattyacids, alkylbenzene sulphonic acids, alkane sulphonic acids, linearalpha-olefin sulphonic acid or the alkaline earth metal salts of any ofthe above acids, phospholipids such as lecithin, synthetic polymers suchas Hypermer OM-1 (trademark of ICI).

The colloidal particles comprise one or more materials selected from butnot limited to barium sulphate (barite), calcium carbonate, dolomite,ilmenite, hermatite or other iron ores, olivine, siderite, strontiumsulphate. Normally the lowest wellbore fluid viscosity at any particulardensity is obtained by using the highest density colloidal particles.However other considerations may influence the choice of product such ascost, local availability and the power required for grinding.

Calcium carbonate and dolomite posses the advantage that residual solidsor filter cake may be readily removed from a well by treatment withacids.

This invention has a surprising variety of applications in drillingfluids, cement, high density fluids and coiled tubing drilling fluids tohighlight a few. The new particulate weighting agents have the abilityto stablise the laminar flow regime, and delay the onset of turbulence.It is possible to formulate fluids for several applications includingcoiled tubing drilling fluids, that will be able to be pumped fasterbefore turbulence is encountered, so giving essentially lower pressuredrops at equivalent flow rates. This ability to stabilise the laminarflow regime although surprising, is adequately demonstrated in heavydensity muds of 20 pounds per gallon (2.39 g/cm³) or higher. Such highdensity muds using conventional weighting agents with a weight averageparticle diameter of 10 to 30 μm would exhibit dilatancy with theconcomitant increase in the presence drops due to the turbulencegenerated. The ability of the new weighting agent to stabilise the flowregime even in the presence of a component of larger particles, meansthat high density fluids with acceptable rheology are feasible withlower pressure drops.

A further and unexpected application occurs in cement whereby the newweighting agent will generate slurries of a more controlled and lowerrheology so allowing it to be pumped more freely into position. Thereduced particle size will tend to have a less abrasive nature, whilstits suspension characteristics will reduce the free water and othersuspension issues encountered when setting the cement. The high fractionof fines should also act as efficient fluid loss control agents, sopreventing gas migration and producing stronger cements.

The fluids of the present invention may also be used in non-oilfieldapplications such as dense media separating fluid (to recover ore forexample) or as a ship's ballast fluid.

The following examples are to illustrate the properties and performanceof the wellbore fluids of the present invention though the invention isnot limited to the specific embodiments showing these examples. Alltesting was conducted as per API RP 13 B where applicable. Mixing wasperformed on Silverso L2R or Hamilton Beach Mixers. The viscosity atvarious shear rates (RPM's) and other rheological properties wereobtained using a Fann viscometer. Mud weight were checked using astandard mud scale or an analytical balance. Fluid loss was measuredwith a saturated API fluid loss cell.

In expressing a metric equivalent, the following U.S. to metricconversion factors are used: 1 gal=3.785 liters; 1 lb.=0.454 kg; 1lb./gal (ppg)=0.1198 g/cm³; 1 bbl=42 gal; 1 lb./bbl (ppb)=2.835 kg/m³; 1lb/100 ft²=0.4788 Pa.

These tests have been carried out with different grades of barite: astandard grade of API barite, having a weight average particle diameter(D₅₀) of about 20 μm; a commercial barite (M) made by milling/grindingbarite whilst in the dry state, with an average size of 3μ-5μ andcolloidal barite according the present invention (with a D₅₀ from 0.5 μmto 1.5 μm), with a dipsersant included during the “wet” grindingprocess. The corresponding particle size distributions are shown FIG. 1.The dispersant is IDSPERSE™ XT (Mark of Schlumberger), an anionicacrylic ter-polymer of molecular weight in the range 40,000-120,000 withcarboxylate and other functional groups. This preferred polymer isadvantageously stable at temperature up to 200° C., tolerant to a broadrange of contaminant, gives good filtration properties and do notreadily desorb off the particle surface.

EXAMPLE 1

22 ppg [2.63 g/cm³] fluids based on barium sulphate and water wereprepared using standard barite and colloidal barite according to theinvention. The 22 ppg slurry of API grade barite and water was made withno gelling agent to control the inter-particle interactions (Fluid #1).Fluid #2 is also based on standard barite but with a post-addition oftwo pounds per barrel (5.7 kilograms per cubic meter) IDSPERSE XT. Fluid#3 is 100% new weighting agent with 67% of particles below 1 micron insize and at least 90% less than 2 μm. The results are provided in tableI.

TABLE I Viscosity at various shear rates (rpm of agitation): Dialreading Yield or “Fann Units” for: Plastic Point 600 300 200 100Viscosity lb/100 ft² # rpm rpm rpm rpm 6 rpm 3 rpm mPa.S (Pascals) 1 250160 124 92 25 16 90 70 (34) 2 265 105 64 26 1 1 160 −55 (−26) 3 65 38 2717 3 2 27 11 (5) 

For Fluid #1 the viscosity is very high and the slurry was observed tofilter very rapidly. (If further materials were added to reduce thefluid loss, the viscosity would increase yet further). This system sagssignificantly over one hour giving substantial free water (ca. 10% oforiginal volume).

Post addition of two pounds per barrel [5.7 kg/cm³] of IDSPERSE XT tothis system (Fluid #2) reduces the low shear rate viscosity bycontrolling the inter-particle interactions. However due to the particleconcentration and average particle size the fluid exhibits dilatencywhich is indicated by the high plastic viscosity and negative yieldpoint. This has considerable consequences on the pressure drops forthese fluids whilst pumping. The fluid #2 sags immediately on standing.

By contrast, Fluid #3 exhibits an excellent, low, plastic viscosity. Thepresence of the dispersing polymer controls the inter-particleinteractions, so making fluid #3 pumpable and not a gel. Also the muchlower average particle size has stabilised the flow regime and is nowlaminar at 1000 s⁻¹ demonstrated by the low plastic viscosity andpositive yield point.

EXAMPLE 2

Experiments were conducted to examine the effect of the post addition ofthe chosen polyemr dispersant to a slurry comprising weighting agents ofthe same colloidal particle size. A milled barite (D₅₄-4 um) and acomminuted Calcium carbonate (70% by weight of the particles of lessthan 2 μm) were selected, both of which are of similar particle size tothe invention related herein. The slurries were prepared at anequivalent particle volume fraction of 0.282. See table II.

The rheologies were measured at 120° F. (49° C.) thereafter an additionof 6 ppb (17.2 kg/m³) IDSPERSE XT was made. The rheologies of thesubsequent slurries were finally measured at 120° F. (see table III)with additional API fluid loss test.

TABLE II Volume # Material Dispersant Density (ppg) Fraction wt/wt 4 NewBarite while grinding 16.0 [1.92 g/cm³] 0.282 0.625 5 Milled Barite none16.0 [1.92 g/cm³] 0.282 0.625 6 Milled Barite post-addition 16.0 [1.92g/cm³] 0.282 0.625 7 Calcium none 12.4 [1.48 g/cm³] 0.282 0.518Carbonate 8 Calcium post-addition 12.4 [1.48 g/cm³] 0.282 0.518Carbonate

TABLE III Viscosity at various shear rates (rpm of agitation) : PlasticYield API Dial reading or “Fann Units” for: Viscosity Point Fluid # 600rpm 300 rpm 200 rpm 100 rpm 6 rpm 3 rpm mPa.s lb/100 ft² Loss 4 12 6 4 26 0 11 5 os os os os os os 6 12 6 4 2 6 0 total¹ 7 os os 260  221  88 788 12 6 4 3  1  1 6 0 total² ¹- total fluid loss in 26 minutes ²- totalfluid loss in 20 minutes

No filtration control is gained from post addition of the polymer asrevealed by the total fluid loss in the API test.

EXAMPLE 3

This test was carried out to show the feasibility of 24 ppg [2.47 g/cm³]slurries (0.577 Volume fraction). Each fluid contained the followingcomponents e.g. Fresh Water 135.4 g, Total Barite 861.0 g, IDSPERSE XT18.0 g. The barite component was varied in composition according to thefollowing table.

TABLE IV API grade Colloidal # Barite (%) Barite (%)  9 100   0 10 90 10 11 80  20 12 75  25 13 60  40 14  0 100

TABLE V Yield Viscosity at various shear rates (rpm of agitation) : DialPlastic Point reading or “Fann Units” for: Viscosity lb/100 ft² # 600300 200 117 100 59 30 6 3 mPa.s (Pascals) 9 *os 285 157 66 56 26 10 3 210 245 109 67 35 16 13 7 3 2 136 −27 (−13) 11 171 78 50 28 23 10 7 3 293 −15 (−7)  12 115 55 36 19 17 8 5 3 2 60 −5 (−2) 13  98 49 34 21 20 1410 4 3 49 0 14 165 84 58 37 32 22 18 5 3 81  3 (−1.5) *os = off-scale

The results provided table V show that API grade barite due to itsparticle size and the high volume fraction required to achieved high mudweights exhibit dilatancy i.e. high plastic and apparent viscosity andnegative yield values.

Introduction of fine grade materials tends to stablise the flow regimekeep it laminar at higher shear rates: plastic viscosity decreasesmarkedly and yield point changes from negative to positive. Nosignificant increase in low-shear rate viscosity (@3 rpm) is caused bythe colloidal barite.

These results show that the colloidal weight material of this inventionmay advantageously be used in conjunction with conventional API barite.

EXAMPLE 4

An eighteen (18) pound per gallon [2.15 g/cm³] slurry of weighting agentaccording the present invention was formulated and subsequentlycontaminated with a range of common contaminants and hot rolled at 300°F. (148.9° C.). The rheological results of before (BHR) and after hotrolling (AHR) are presented below. The system shows excellent resistanceto contaminants, low controllable rheology and gives fluid loss controlunder a standard API mud test as shown in following table VI: Anequivalent set of fluids were prepared using API conventional baritewithout the polymer coating as a direct comparison of the two particletypes. (Table VII)

TABLE VI (New barite) Viscosity (Fann Units) at various YP Fluid shearrates (rpm of agitation): PV lb/100 ft² loss 600 300 200 100 6 3 mPa.s(Pascals) ml no contaminant BHR 21 11 8 4 1 1 10   1(0.5) no contaminantAHR 18 10 7 4 1 1 8 2(1) 5.0 +80 ppb NaCl BHR 41 23 16 10 2 1 18  5(2.5) +80 ppb NaCl AHR 26 14 10 6 1 1 12 2(1) 16 +30 ppb OCMA¹ BHR 3822 15 9 2 1 16 6(3) +30 ppb OCMA AHR 26 14 10 6 1 1 12 2(1) 6.8 +5 ppbLime BHR 15 7 5 3 1 1 8   −1(−0.5) +5 ppb Lime AHR 10 5 4 2 1 1 5 0 6.4¹OCHMA = Ocma clay, a fine particle ball clay commonly used to replicatedrilled solids contamination

TABLE VII (Conventional API Barite) Viscosity (Fann Units) at various YPFluid shear rates (rpm of agitation): PV lb/100 ft² loss 600 300 200 1006 3 mPa.s (Pascals) ml no contaminant BHR 22 10 6 3 1 1 12 −2 nocontaminant AHR 40 24 19 11 5 4 16 8 Total¹ +80 ppb NaCl BHR 27 13 10 62 1 14 −1 +80 ppb NaCl AHR 25 16 9 8 1 1 9 7 Total¹ +30 ppb OCMA BHR 6955 49 43 31 26 14 31 +30 ppb OCMA AHR 51 36 31 25 18 16 15 21 Total² +5ppb Lime BHR 26 14 10 6 2 1 12 2 +5 ppb Lime AHR 26 14 10 6 1 1 12 2Total¹ ¹ - Total fluid loss within 30 seconds ² - Total fluid losswithin 5 minutes.

A comparison of the two sets of data show that the weighting agentaccording the present invention has considerable fluid loss controlproperties when compared to the API barite. The API barite also showssensitivity to drilled solids contamination whereas the new baritesystem is more tolerant.

EXAMPLE 5

An experiment was conducted to demonstrate the ability of the newweighting agent to formulate drilling muds with densities above 20 poundper gallon [2.39 g/cm³].

Two twenty two pound per gallon [2.63 g/cm³] mud systems wereformulated, the weighting agents comprised a blend of 35% w/w new bariteweighting agent with 65% w/w API grade Barite (Fluid #1) weighting agentand 100% API grade barite (fluid #2), both with 11.5 pound per barrel[32.8 kg/m³] STAPLEX 500 (mark of Schlumberger, shale stabiliser), 2pound per barrel [5.7 kg/m³] IDCAP (mark of Schlumberger, shaleinhibitor), and 3.5 pound per barrel [10 kg/m³] KCl. The other additivesprovide inhibition to the drilling fluid, but here demonstrate thecapacity of the new formulation to cope with any subsequent polymeradditions. The fluid was hot rolled to 200° F. (93.3° C.). Results areprovided in table VIII.

TABLE VIII Yield Viscosity (Fann Units) at various Point Fluid shearrates (rpm of agitation): PV lb/100 ft² loss 600 300 200 100 6 3 mPa.s(Pascals) ml Before Hot Rolling (#1)  10  58 46 30 9 8 52   6 (2.9)After Hot Rolling (#1) 123  70 52 30 9 8 53  17 (8.1) 8.0 Before HotRolling (#2) 270 103 55 23 3 2 167 −64 (−32) After Hot rolling (#2) os177 110 47 7 5 12.0 os : off-scale

The 100% API grade barite has very high plastic viscosity and is in factturbulent as demonstrated by the negative yield point, after hot rollingthe rheology is so high it is off scale.

EXAMPLE 6

This experiment demonstrates the ability of the new weighting agent tolow viscosity fluids. The weighting agent is 100% colloidal bariteaccording the present invention. Fluid #15 is based on a pseudo-oil(Ultidrill, Mark of Schlumberger, a linear alpha-olefin having 14 to 16carbon atoms). Fluid #16 is a water-based mud and includes a viscosifier(0.5 ppb IDVIS, Mark of Schlumberger, a pure xanthan gum polymer) and afluid loss control agent (6.6 ppb IDFLO Mark of Schlumberger). Fluid #15was hot rolled at 200° F. (93.3° C.), fluid #16 at 250° F. (121.1° C.).After hot rolling results are shown table IX.

TABLE IX Yield Viscosity (Fann Units) at various Gels¹ Point shear rates(rpm of agitation): PV lbs/100 ft² lbs/100 ft² 600 300 200 100 6 3 mPa.s(Pascals) (Pascals) #15 : 13.66 ppg 39 27 23 17 6 5 12 7/11 15 [1.63g/cm³] #16 : 14 ppg 53 36 27 17 6 5 17 5/— 19 [1.67 g/cm³] ¹A measure ofthe gelling and suspending characteristics of the fluid, determined at10 sec/10 min using a Fann viscosimeter.

Even though the formulation was not optimized, this test makes clearthat the new weighting agent provides a way to formulate brine analoguesfluids useful for slimhole applications or coiled tubing drillingfluids. The rheology profile is improved by the addition of colloidalparticles.

EXAMPLE 7

An experiment was conducted to demonstrate the ability of the newweighting agent to formulate completion fluids, were density control andhence sedimentation stability is a prime factor. The weighting agent iscomposed of the new colloidal barite according to the present inventionwith 50 pound per barrel [142.65 kg/m²] standard API grade calciumcarbonate which acts as bridging solids. The 18.6 ppg [2.23 g/cm³] fluidwas formulated with 2 pound per barrel [5.7 kg/m³] PTS 200 (mark ofSchlumberger, pH buffer) The static ageing tests were carried out at400° F. (204.4° C.) for 72 hours. The results shown in the table below,before (BSA) and after (ASA) static ageing reveal good stability tosedimentation and rheological profile.

Viscosity (Fann Units) at various YP Free shear rates (rpm ofagitation): PV lb/100 ft² water * 600 300 200 100 6 3 mPa.s (Pascals) ml18.6 ppg BSA 37 21 15 11 2 1 16 5 (2.5) — 18.6 ppg ASA 27 14 11  6 1 113 1 (0.5) 6 * free water is the volume of clear water that appears ontop of the fluid. The remainder of the fluid has uniform density.

EXAMPLE 8

The experiment demonstrates the ability of the new weighting agent toformulate low viscosity fluids and show it's tolerance to pH variations.The weighting agent is composed of the new colloidal barite according tothe present invention. The 16 ppg [1.91 g/cm³] fluid was formulated withcaustic soda to adjust the pH to the required level, with the subsequentfluid rheology and API filtration tested. The results shown in the tablebelow reveal good stability to pH variation and rheological profile.

Yield Viscosity (Fann Units) at various Point Fluid shear rates (rpm ofagitation): PV lbs/100 ft² Loss PH 600 300 200 100 6 3 mPa.s (Pascals)ml 8.01 14 7 5 3 7 0 (0)   8.4 9.03 14 8 5 3 6 2 (1)   8.5 10.04 17 9 63 8 1 (0.5) 7.9 10.97 17 9 6 3 8 1 (0.5) 7.9 12.04 19 10 7 4 1 1 9 1(0.5) 8.1

EXAMPLE 9

This experiment demonstrates the ability of the new weighting agent toformulate low rheology HTHP water base fluids. The weighting agent iscomposed of the new colloidal barite according to the present invention,with 10 pounds per barrel [28.53 kg/m³] CALOTEMP (mark of Schlumberger,fluid loss additive) and 1 pound per barrel [2.85 kg/m³] PTS 200 (markof Schlumberger, pH buffer). The 17 ppg [2.04 g/m³] and 18 ppg [2.16g/cm³] fluids were static aged for 72 hours at 250° F. (121° C.). Theresults shown in the table below reveal good stability to sedimentationand low rheological profile with the subsequent filtration tested.

Yield Viscosity (Fann Units) at various Point Free Fluid Density shearrates (rpm of agitation): PV lbs/100 ft² Water Loss ppg PH 600 300 200100 6 3 mPa.s (Pascals) ml ml 17 7.4 28 16 11  6 1 1 12 4 (2) 10 3.1 187.5 42 23 16 10 1 1 19 4 (2)  6 3.4

What is claimed is:
 1. An additive composition comprising solidcolloidal particles of weight average particle diameter (d₅₀) of lessthan 2 μm but not more than 5% of the particles are less than 0.1 μm indiameter, and a dispersant in an amount sufficient to disperse theparticles in a wellbore fluid further comprising a liquid medium inwhich the liquid medium comprises an organic liquid.
 2. A wellbore fluidcomposition comprising an additive for increasing the density of afluid, said additive prepared by comminuting a solid material to solidcolloidal particles of weight average particle diameter (d₅₀) of lessthan 2 μm but not more than 5% of the particles are less than 0.1 μm indiameter, in the presence of a dispersant, which is prepared by addingthe additive in a liquid medium to a wellbore fluid, in which the liquidmedium is an organic liquid of kinematic viscosity less than 10centistokes (10 mm²/s) at 40° C. and of flash point of greater than 60°C.
 3. An additive composition comprising solid colloidal particles ofweight average particle diameter (d₅₀) of less than 2 μm but not morethan 5% of the particles are less than 0.1 μm in diameter, a dispersantin an amount sufficient to disperse the particles in a wellbore fluid,and an organic liquid medium.
 4. An additive for increasing the densityof a wellbore fluid, said additive consisting essentially of solidcolloidal particles of weight average particle diameter (d₅₀) of lessthan 2 μm but not more than 5% of the particles are less than 0.1 μm indiameter, and a dispersant, the particles being dispersed in the fluidby the action of said dispersant present in an amount sufficient todisperse the particles.
 5. A method of preparing a wellbore fluidcomposition comprising providing a wellbore fluid; adding to thewellbore fluid a liquid medium consisting essentially of solid colloidalparticles of weight average particle diameter (d₅₀) of less than 2 μmbut not more than 5% of the particles are less than 0.1 μm in diameter,and a dispersant, and dispersing the solid colloidal particles in thewellbore fluid wherein dispersant is selected from carboxylic acids ofmolecular weight of at least 150, polybasic fatty acids, alkylbenzenesulphonic acids, alkane sulphonic acids, linear alpha-olefin sulphonicacid or the alkaline earth metal salts of any of the above acids, andphospholipids.
 6. The method of claim 5 wherein the carboxylic acid isoleic acid.